There has been a continuing desire in the electronics industry to obtain higher circuit density in microelectronic devices. To achieve the higher degree of integration necessary to meet the ground rules for such devices, lithographic techniques have been subjected to improvements which enhance the resolution of lithographic patterns in photoresist films.
Aqueous base developable photoresist compositions which crosslink through electrophilic aromatic substitution have been disclosed in Reck et al., SPIE Regional Technical Conference on Photopolymers, Ellenville, N.Y., 63 (1988). Such formulations are characterized by having 3 structural components which include (a) an aromatic moiety, such as poly(4-hydroxystyrene) or novolak, which is susceptible to electrophilic aromatic substitution and allows for development under ionizing conditions, (b) a stable substance which is capable upon reaction with acid, of generating an electrophile, viz., a carbocationic species whose precursor is the acetate of an aryl methylol, and (c) a photo-acid generator which is an onium salt such as triphenylsulfonium hexafluoroantimonate. Upon exposure latent images are formed consisting of a strong acid dispersed in the exposed areas of the resist film. When the film is baked briefly after exposure, the acid cleaves the acetate group of the latent electrophile to produce a benzylic carbocationic species which reacts immediately with the surrounding phenolic resin to form new carbon-carbon bonds. Such formation of new carbon-carbon bonds results in an increase in molecular weight of the resin or its crosslinking, thereby drastically altering its solubility properties. Subsequent development in dilute base affords a negative-tone image of the mask. (Systems which included novolak resins were said to exhibit an unacceptable high optical density in deep UV).
In Schellekens et al., Proc. SPIE 1086, 220 (1989), single layer resist systems are provided that involve the gas phase silylation and dry development& of diazoquinone novolak resists and poly(vinylphenol) resists in the novolak case a novolak identified as AZ-Protective Coating was used and 2,6-bis(hydroxymethyl)-4-t-butyl-1-hydroxybenzene was used as a crosslinking agent and diphenyliodonium hexafluorophosphate was used as the deep UV sensitizer. In the case of the poly(vinylphenols), hexamethoxymethylmelamine was used as the crosslinker and 2(4-phenylphenyl)-4,6-bis(trichlorolmethyl)-s-triazine was used as the sensitizer. In contrast to previously known esterification crosslinking schemes, the authors of this paper took as the position that the crosslinking both in the case of the novolak species and in the case of the poly(vinylphenols) occurs due to a transetherification mechanism. The crosslinking sites are the hydroxy groups on the polymer chain. This paper further discloses the silylation of the resists using both hexamethyl disilazane and trimethylsilyldimethylamine as silylating agents.
U.S. Pat. No. 4,613,398 to Chiong et al. is directed to methods for formation of etch resistant resists through the preferential permeation of organometallic materials into the differentially exposed polymeric resist materials. In certain embodiments, the differential permeation is achieved through the crosslinking of the polymer molecules to increase the molecular weight of the polymer and reduce its permeability. Typical of the photocrosslinkable polymers are those which contain functional groups such as hydroxy, carbonyl, phenol, amine and imide NH. These polymers include novolak resins, resoles, epoxides, and polymers containing azido groups and polyviynylphenol additionally crosslinking maybe through polyfunctional monomers and prepolymers containing double bonds and epoxy groups.
U.S. Pat. No. 4,810,601 to Allen et al. is concerned with the formation of top surface images in resist films that transfers the image to only the top surface of the film so that the subsequent treatment with organometallic reagent occurs only in the top fraction of the film which is more permeable or more reactive to the organometallic reagent giving a structure which is equivalent to a bi or multilayer system. That disclosure is directed primarily to those materials which have recurrent acid labile pendent groups that when released from the polymer chain provide sights for reaction with the organometallic material.
U.S. patent application Ser. No. 07/735,420, filed Jul. 25, 1991 to Allen et al., is directed to methods for converting chemically amplified photoresist systems to a top surface imaging resist system by the incorporation of an opaquing dye in the photoresist composition to prevent the passage of imaging radiation through a substantial portion of the film and treating the imaged film with a silylating agent and dry developing such treated film to form negative tone resist images.